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mindburner

power supply / circuit gurus, please help

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Hi All,

I am new here, having just joined, but have been lurking on the forum for some time.

I have been following this thread with interest (as a retired electronics engineer) and I thought a couple of things may have been overlooked in the change over to the LF1083.

Firstly: - have you made sure that the TAB of the new regulator is isolated properly from the heatsink?... especially if the heatsink is attached to the project case, which may be either at 0v and/or earth.... this would short out the output pin and prevent the regulator from turning on.

(note... you may not get a big bang in such a case as the internal short circuit limiting would prevent the output from passing current.

Secondly: - The LF1083 requires a more strict bias regime than the LM338 in that it must have at the very minimum 5mA of bias current (10mA would be best).

It also requires a minimum load of 10mA.

The 300 Ohm resistor in your existing circuit would only provide 4.16ma of bias current.

If you change this resistor to 100 Ohms this would provide 13.75mA of bias current and also provide the minimum load current.

The control pot (10K at present) is far to large to be used with a 100 Ohm bias resistor and would make setting the output voltage very touchy (sensitive)... A value of 1k Ohms - 2K Ohms would be more appropriate and would make setting easier.

Hope this helps.

Best regards.

Sandy.

hi and welcome to the forum. Thats great info there. I appreciate you doing the calculations. I have just been basing away in great expectation:)

The heatsink is attached to the hard plastic case and there is a mica insulator under the regulator so it should be ok.

I have just got some 1k pots so will change the pot and resistor and see how it goes.

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That diagram is out of the datasheet, there a few other examples as well. Personally i would do what Sandy said. Use a isolating washer to mount the device and swap the pot for a 1K and resistor to 100 ohms. This would be easy and probably the cheapest option seeing your almost there.

Simon

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Hi again mindbender,

No problem, pleased to be of some help.

Ok then, even though you have a mica washer under the regulator these are prone to failure when changing things about... there should also be 2 plastic bushes to isolate the bolt passing through the device and heatsink.

If you have a multimeter available then it is easy to check for a short... if you put the multimeter in Ohms mode and check between the TAB on the regulator and the heatsink/0v you should get a highish reading...1 kOhms or more... if you get a low or zero reading then check the bushes and mica washer for damage.

One other check to make is to see if you have cracked the circuit board track between the output pin on the regulator and the nearest other component (probably the 300 Ohm resistor)

It is easy to do this when you un-solder a connection to replace a component.

You should get a zero reading when doing this test

Hopefully it is just the change to a 100 Ohm resistor and change of pot that will get you up and running again.

NOTE** make sure the circuit is powered down before making the above checks.

Just out of interest... what is your maximum load current requirement?

Best regards.

Sandy.

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Hi again mindbender,

No problem, pleased to be of some help.

Ok then, even though you have a mica washer under the regulator these are prone to failure when changing things about... there should also be 2 plastic bushes to isolate the bolt passing through the device and heatsink.

If you have a multimeter available then it is easy to check for a short... if you put the multimeter in Ohms mode and check between the TAB on the regulator and the heatsink/0v you should get a highish reading...1 kOhms or more... if you get a low or zero reading then check the bushes and mica washer for damage.

One other check to make is to see if you have cracked the circuit board track between the output pin on the regulator and the nearest other component (probably the 300 Ohm resistor)

It is easy to do this when you un-solder a connection to replace a component.

You should get a zero reading when doing this test

Hopefully it is just the change to a 100 Ohm resistor and change of pot that will get you up and running again.

NOTE** make sure the circuit is powered down before making the above checks.

Just out of interest... what is your maximum load current requirement?

Best regards.

Sandy.

hi double checked continuity at the reg and it's ok. I am starting to believe that the LT1083 was faulty to begin with. The heat sink is isolated from earth or +VE and I was very careful when connecting the reg to ensure it was done correctly. It was an ebay special at a third the cost of other suppliers. I have ordered another from a different source, so wlll have a go with that. I also have insulators etc which I will fit to the new reg.

The PCB is fin as it works ok with the LM338, just not the LT1083.

The transformer I am using has deformed the ABS case i'm using with heat, not good. I had to mount it flat as the case was just not tall enough to take it chassis style.

it's a DP241-8-36 36 VCT @ 2.8A

I am using one of the taps from it, may be better using both taps?

The total current is 4.3A and is pretty stable

I may resort to my two sealed switched mode PSU's as this tranny heat issue has me worried a bit but I may be able to get a better transformer, even a torodial, but wanted to keep the cost down a bit.

Had a quick go at m42 with the cooled and modded camera using my rather inefficientl power supply. Got a few 5 mins subs of M42 at about -4 deg EXIF

It's not amazing but I was please the camera still works:) after all my messing around.

post-11156-0-83703900-1360429176_thumb.p

Edited by mindburner

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That diagram is out of the datasheet, there a few other examples as well. Personally i would do what Sandy said. Use a isolating washer to mount the device and swap the pot for a 1K and resistor to 100 ohms. This would be easy and probably the cheapest option seeing your almost there.

Simon

hi yes, I have just swapped the resistor out. Still using the 10k pot and adjustment is messy, but didn't sort out the regulation, the LM1083 is a deceased regulator it seems

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Hi mindburner,

OK then... from what you have tried and established It would certainly suggest that your regulator is a DUFF ONE.

On the heat side of things: -

I am not surprised that your transformer is getting very hot... it is only rated for 2.8Amps at the full 36v output.

If you use the centre tap you will reduce this to 1.4Amps... yes it will probably supply the 4.3Amps you are drawing but transformer regulation will be all over the place... and it will get very hot... and will certainly give your regulator a lot of headache's trying to keep up... creating yet more heat.

You need to replace it with a more suitable unit before you end up with a fire and possible injury to yourself... that is not what is required.

Rapid electronics have a couple of suitable ones.

75VA 230v primary with 2 secondaries.

part no. 88-3447 has 2 x 9v secondaries which can be joined in series to give you 18v.

part no. 88-3449 has 2 x 15v secondaries which can be used in parallel giving 15v.

Either will supply around 5Amps and not get hot and a very good price at the moment.

http://www.rapidonline.com

Do not try using the full secondary on your existing transformer... this will give you 36v out which is too high for your LF1038 regulator (max is 30v) and can still only provide 2.8Amps without overheating.

Sorry to be the bearer of bad news. :huh:

On a lighter note... thats a very nice image you have captured of M42.

Best regards.

Sandy.

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hi Sandy

yes I have been overloading the poor old transformer. I have an RS account and spotted this

75VA with 2x9v taps

which seems to be similar spec as one of the rapid electronics items.

I will order it and at least remove that heating problem.

Thanks for the comment on the image. As I have the camera apart so many times, I was worried that the image quality may have been affected. I also wanted to test the cooling in the open air. I managed to get a drop of about 20 degrees on the sensor, which should reduce the image noise dramatically.

I'll post back how I get on once the regulator and new transformer arrive next week

cheers and thanks

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